Dot 3 and Dot 4 Brake Fluid?

[Ivan4x4]

Hello my fellow Hiluxers.

Please lend me your ears ..or brake knowledge for that matter...my Hilux is suppose to use DOT3 brake fluid according to manual but when I went to buy some, the gentleman explained that you dont get anymore dot 3 brake fluid....so now..... do I just add dot4 to dot 3 brake fluid or should I drain the dot 3 brake fluid and then add dot 4 ??

My Hilux brake system seems to brake less and less..When I brake I do get pressure from foot pedal. I changed the front brake pads on Hilux about 3 months ago."hulle was kla soos ou toust"... it's still not as susceptible as I would like it to be with new brakes..

And how often should you change the Brake fluid??

[CasKru]

Brake fluid does not need to be changed that often but it is a good idea to do it more often than not. Make sure you don't have leak in the system and bleed the system as well. If you don't feel any pressure on the pedal you either have air in the system or a serious leak. If any of you wheels have a oily substance on the inside of the wheel you most probably have a leak.

Brake fluids.
havoline brake fluid

As mentioned elsewhere on the page, brake fluid does not compress. It's a good job too - if you put your foot on the brake pedal and it went all the way to the floor, you'd be worried. But that's exactly what can happen if you disregard the "health" of your brake fluid.
Brake fluid is hygroscopic - that means it attracts and soaks up water. This is why it comes in sealed containers when you buy it, and why when the crazy guy four doors down offers you some of the 15 gallons of brake fluid he's had in his garage since the war, you should turn him down. The problem with it being hygroscopic is that if it does start to take on water, Bad Things can happen. Pull up a chair and allow me to explain.
Your typical DOT 4 brake fluid (see later for DOT ratings) boils at about 446°F (230°C). Water boils at 212°F (100°C). Imagine your brakes are getting hot because of a long downhill stretch. Whilst the brake fluid is quite OK, the temperature of the brake components might get up over the boiling point of water. If that happens, the water boils out of the brake fluid and forms steam - a compressible gas. Next time you put your foot on the brake, rather than braking, all the pressure in the brake system is taken up with compressing the steam. Your brakes go out, you don't stop.
Getting a little more complex, the boiling point of a liquid goes up with its pressure (Physics 101). So when you step on the brake, the boiling point of the brake fluid might actually go up to 500°F (260°C) and the boiling point of the water content might raise up to 250°F (121°C). This is great, you might think, because now the boiling point is higher than the temperature of the brake fluid. At least it is until you take your foot off the brake again. Now the pressure in the system returns to normal, the boiling points revert to normal and instantly the water boils off into steam again. The symptoms are slightly different now. Under this scenario, the brakes work the first one or two times, but on the third or fourth press, they stop working because now the temperature and pressures have conspired to boil the water.
The worst possible scenario is brake-fade (see right at the top) combined with air in the system. If this has happened to you, then you're likely reading this page from beyond the grave, because in most accidents where weak brakes become no brakes, there aren't any survivors.
D.O.T ratings

All brake fluids are DOT rated. Your owners handbook for your car or motorbike probably tells you to use DOT3 or DOT4 from a sealed container. The DOT ratings are a set of minimum standards the fluid must adhere to in order to get the rating, and thus work in your braking system. The following table shows the various properties of DOT ratings. Remember that the values here are the minimum values. Most manufacturers make sure their product far exceeds minimum ratings.
Boiling Point DOT 3 DOT 4 DOT 5 (silicone-based) DOT 5.1 (non-silicone based)
Dry 401°F 446°F 500°F 500°F
Wet 284°F 311°F 365°F 365°F

The "dry" and "wet" boiling points in the table above are for brake fluid which is fresh from the bottle (dry) and which has a 10% water content (wet). A DOT study in 2000 discovered that on average, the brake fluid in a vehicle absorbs about 2% water every 12 months.
The two types of brake fluids shown in the table are DOT3/DOT4/DOT5.1 which are glycol (Polyalkylene Glycol Ether) based, and DOT5 which is silicone based. DOT3 and DOT4 fluids are interchangeable* - the only real difference is their boiling point. Theoretically you could interchange DOT4 and DOT5.1 fluids too but I wouldn't recommend it. DOT3/4/5.1 and DOT5 fluids cannot be mixed or interchanged under any circumstances. They mix like oil and water (ie. they don't) and the silicon based fluids can destroy the seals in brake systems which rely on the moisturiser additives that are present in DOT3/4/5.1 fluids.
Other things you ought to know about silicone based fluids:
- they are resistant to absorbing water, which is why their wet boiling points are so high. Problem is that any water content eventually pools in the low spots of the brake system and causes rust.
- they don't strip paint.
- they are not compatible with most ABS system because they doesn't lubricate the ABS pump like a glycol based fluid.
- putting this fluid in systems which have had DOT3/4 fluid in will cause the seals in the caliper and master cylinders to malfunction. Which means they need replacing. Which is expensive.

Oh, and don't ask me why DOT5.1 is glycol and DOT5 is silicon based. It doesn't make and sense to me either.

* There has been some discussion as to the use of DOT4 fluid in Toyotas that recommend DOT3 fluid - apparently something in the Toyota braking system doesn't play well with DOT4 fluid, particularly the master brake cylinder seals.

[Ivan4x4]

Thanks gor your quick reply..... I do have pressure on pedal just not enough stopping power...Could it be the brake disks that is "kla" ? And how do I know if the disks is kaput?

[CasKru]

Thanks gor your quick reply..... I do have pressure on pedal just not enough stopping power...Could it be the brake disks that is "kla" ? And how do I know if the disks is kaput?

Look at the calliper from the side. You should see at least 2 to 3mm of the compound. It could also be that your brakes over heated at some stage which could have cause glazing.. similar to when you burn your clutch

[mplester]

Dot 3 & 4 are used on vehicels with boosters. Dot 5 is used on race cars.
You should completely flush the brake system with new brake fluid every 2 years. This is due to water contamination. This is aslo the same reason why when you open a bottle of brake fluid it should be used ASAP.
The water contamination causes corrosion in the brake system & lowers the flash point of the fluid.
Do not mix different specs of fluid.

[Ivan4x4]

Thanks for info ...wil do it ASAP